This invention involves apparatus and a method of draining slag caused by condensation of volatiles from furnace exhaust gases from the bottom of a furnace stack and is particularly useful on oxygen boosted and more particularly on oxy-fuel fired furnaces, e.g. oxy-fuel fired glass melters.
Furnaces fired with air-fuel typically exhaust large quantities of hot gases. Frequently, such as with glass furnaces, the hot exhaust gases contain inorganic volatiles such as sodium borate, etc. As the hot gases cool going up a stack, through a recuperator and stack, or through a regenerator and stack, some of the volatiles will condense into a liquid on the cooler, but still hot surfaces. This low melting point liquid of condensed volatiles drains down the walls of the stack, etc. and collects in a reservoir in the base or bottom portion of the stack. Periodically, it is necessary to clean out this reservoir to prevent this liquid from flowing back into the furnace through the exhaust port which causes a glass composition shift and furnace upset. Draining this slag, which also contains batch carryover, some refractory particles, and some dissolved refractory, is a difficult, hot and unpleasant job, requiring about 8 hours, more or less depending on the particular unit, for two men.
With air-gas firing, cleaning out the reservoir is necessary only infrequently because the stack is normally large in cross section to accommodate the large volume rate of exhaust and the reservoir is usually made deep to hold a lot of slag. A typical E glass air-fuel fired furnace would have a stack with an internal area of at least 3600 square inches and the depth of the reservoir, the distance between the interior of the bottom wall of the exhaust port to the interior of the bottom of the reservoir, would typically be about 36 inches, providing a reservoir having a volume of at least about 129,00 cubic inches or more. A reservoir of this size requires cleaning out about every 5-6 weeks on E glass. When melting soda-lime or sodium borosilicate glasses it is necessary to clean out the reservoir much more frequently because the volatile carry over and condensation is much greater due to the much higher soda content of the batch.
The industry is switching from air-fuel and preheated air-fuel firing to oxygen boosting and oxy-fuel firing to reduce emission problems and to reduce bottom line costs. Oxy-fuel firing is defined here as a firing system and/or process wherein at least 75 percent, preferably at least 85 percent and most preferably at least 90 percent of the oxygen needed for combustion is supplied with oxygen, industrial grade or purer, instead of air. In the most preferred oxy-fuel systems, air is present only unintentionally, i.e. due to leaks in the furnace or as impurities in the oxygen as a result of the oxygen generating system being used.
With oxygen boosting and much more so with oxy-fuel firing, the emission volume rates are lower on the furnace, but the concentration of inorganic volatiles, grams/1000 cubic feet, is higher. Also, when a furnace is converted from air-fuel firing to oxy-fuel firing the stack design and/or the actual stack is often left unchanged. With the lower volume rate of emissions, frequently at lower temperatures too, the emitted gases are cooled to a much lower temperature before they reach very far up the stack than they are in a typical air-fuel fired stack. Hence, the rates of condensation of inorganic volatiles on the walls and the collection rates of resultant slag in the bottom of the stack in an oxy-fuel fired furnace are considerably higher than in air-fuel fired furnaces, e.g. the same size reservoir described above would have to be cleaned out every 2-4 weeks when the firing is converted to an oxy-fuel system.
When the stack is redesigned and rebuilt according to the lower exhaust volume rates it must handle with oxy-fuel firing, the cross section drops to only about 730 square inches in the furnace described above. This reduces the reservoir size to less that one-fourth its previous size and requiring clean out at least 4 times as often. Thus cleaning the slag out of the stack becomes a real headache and a time consuming activity, particularly since with the lower exhaust volume from oxy-fuel firing, the bottom of the stack is cooler and tends to solidify or stiffen the slag making it more difficult to remove. A solution to this problem is definitely needed in the industry.